Combustion engines can be integrated with air separation processes in highly efficient systems for the generation of electric power and the production of atmospheric gas products. A wide variety of fuels can be used in these combustion engines, including natural gas, fuel gas generated by the gasification of liquid or solid carbonaceous materials, and liquid hydrocarbons. Fuel gas generated by gasification typically uses oxygen from the air separation process.
Gas turbine combustion engines, or gas turbines, can be used to drive electric generators in combined cycle generation systems in which the expansion turbine exhaust is used to generate steam which is expanded in a bottoming cycle steam turbine which drives another electric generator. In the production of oxygen and/or nitrogen, the pressurized air feed for the air separation process can be provided partly or completely by the gas turbine compressor, and nitrogen from the air separation process can be introduced into the gas turbine combustor for additional energy recovery and control of NO.sub.x formation.
Comprehensive reviews of integration methods for gas turbines and air separation units are given in a paper entitled "Next-Generation Integration Concepts for Air Separation Systems and Gas Turbines" by A. R. Smith et al in Transactions of the ASME, Vol. 119, April 1997, pp. 298-304 and in a presentation entitled "Future Direction of Air Separation Design for Gasification, IGCC, and Alternative Fuel Projects" by R. J. Allam et al, IChem.sup.E Conference on Gasification, Sep. 23-24 1998, Dresden, Germany.
A common mode of integration between the gas turbine and air separation systems is defined as full air and nitrogen integration. In this operating mode, all air for the gas turbine combustor and the air separation unit is provided by the gas turbine air compressor which is driven by the expansion turbine, and nitrogen from the air separation unit is utilized in the integrated system. Full air and nitrogen integration is described in representative U.S. Pat. Nos. 3,731,495, 4,224,045, 4,250,704, 4,631,915, and 5,406,786, wherein the nitrogen is introduced into the gas turbine combustor. Full air and nitrogen integration also is described in U.S. Pat. Nos. 4,019,314 and 5,317,862, and in German Patent Publication DE 195 29 681 A1, wherein the nitrogen is work expanded to provide work of compression for the air feed or to generate electric power.
The gas turbine and air separation processes can operate in an alternative mode, defined as partial air integration with full nitrogen integration, in which a portion of the air feed for the air separation unit is provided by the gas turbine compressor and the remainder is provided by a separate air compressor driver with an independent power source. Nitrogen from the air separation unit is introduced into the gas turbine combustor or is otherwise work expanded. This operating mode is described in representative U.S. Pat. Nos. 4,697,415; 4,707,994; 4,785,621; 4,962,646; 5,437,150; 5,666,823; and 5,740,673.
In another alternative, nitrogen integration is used without air integration. In this alternative, the gas turbine and air separation systems each has an independently-driven air compressor, and the nitrogen from the air separation unit is returned to the gas turbine combustor. This option is described in representative U.S. Pat. Nos. 4,729,217; 5,081,845; 5,410,869; 5,421,166; 5,459,994; and 5,722,259.
U.S. Pat. No. 3,950,957 and Great Britain Patent Specification 1 455 960 describe an air separation unit integrated with a steam generation system in which a nitrogen-enriched waste stream is heated by indirect heat exchange with hot compressed air from the air separation unit feed air compressor, the heated nitrogen-enriched stream is further heated indirectly in a fired heater, and the final hot nitrogen-enriched stream is work expanded in a dedicated nitrogen expansion turbine. The work generated by this expansion turbine drives the air separation unit main air compressor. The nitrogen expansion turbine exhaust and the combustion gases from the fired heater are introduced separately into a fired steam generator to raise steam, a portion of which may be expanded in a steam turbine to drive the air separation unit feed air compressor. Optionally, the combustion gases from the fired heater are expanded in a turbine which drives a compressor to provide combustion air to a separate fired heater which heats the nitrogen-enriched stream prior to expansion.
An alternative use for high pressure nitrogen from an air separation unit integrated with a gas turbine is disclosed in U.S. Pat. No. 5,388,395 wherein the nitrogen is work expanded to operate an electric generator. The cold nitrogen exhaust from the expander is mixed with the inlet air to the gas turbine compressor thereby cooling the total compressor inlet stream. Alternatively, low pressure nitrogen from the air separation unit is chilled and saturated with water in a direct contact cooler-chiller, and the chilled, saturated nitrogen is mixed with the inlet air to the gas turbine compressor.
U.S. Pat. Nos. 5,040,370 and 5,076,837 disclose the integration of an air separation unit with high-temperature processes which uses oxygen, wherein waste heat from the process is used to heat pressurized nitrogen from the air separation unit, and the hot nitrogen is work expanded to generate electric power.
European Patent Publication EP 0 845 644 A2 describes an elevated pressure air separation unit in which the pressurized nitrogen-enriched product is heated indirectly by the combustion of low pressure fuel in a fired heater, and the hot nitrogen is expanded to produce power or drive gas compressors within the air separation unit.
In the production of oxygen in areas with limited or no accessibility to electric power grids, feed air compression for an air separation unit can be provided by combustion engines if sufficient fuel is available. In industrial operations which require atmospheric gas products such as oxygen, nitrogen, and argon, electric power often is required onsite to drive various types of rotating equipment. When imported electric power is limited, a self-contained process utilizing combustion engine drivers is required to generate the gas products and electric power. The invention disclosed below and defined by the claims which follow addresses this need by providing methods to generate atmospheric gas products and electric power in integrated air separation/combustion engine systems which use alternative methods of recovering and utilizing the heat in the combustion engine exhaust gas stream.